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BAA Observing calendar

Moon at First Quarter

Saturday, 2018, August 18 - 08:50

Dominic Ford – originally published on In-The-Sky.org

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From London (click to change), the Moon will become visible at around 20:39 (BST) as the dusk sky fades, 20° above your south-western horizon. It will then sink towards the horizon, setting 3 hours and 22 minutes after the Sun at 23:37.

The Moon's orbital motion carries it around the Earth once every four weeks, and as a result its phases cycle from new moon, through first quarter, full moon and last quarter, back to new moon once every 29.5 days.

This motion also means that the Moon travels more than 12° across the sky from one night to the next, causing it to rise and set nearly an hour later each day. Click here for more information about the Moon's phases.

The Moon's path in coming days

Over the next few days, the distance between the Moon and Sun will increase each night as the Moon approaches full phase. Rather than rising in the afternoon and appearing high in the sky by sunset, it will rise later and make it less far above your eastern horizon before nightfall. All times are given below in London local time.

Date Sun
sets at
Moon
sets at
Altitude of Moon
at sunset
Direction of Moon
at sunset
13 Aug 2018 20:24 21:46 west
14 Aug 2018 20:22 22:13 13° west
15 Aug 2018 20:20 22:34 17° south-west
16 Aug 2018 20:18 22:59 19° south-west
17 Aug 2018 20:16 23:25 21° south-west
18 Aug 2018 20:14 23:51 21° south-west
19 Aug 2018 20:12 00:24 21° south
20 Aug 2018 20:10 00:24 19° south
21 Aug 2018 20:08 01:01 17° south
22 Aug 2018 20:06 01:44 14° south-east
23 Aug 2018 20:04 02:33 11° south-east
24 Aug 2018 20:02 03:28 south-east

Observing the Moon at first quarter

The period when the Moon shows half phase is ideal for observing the Moon with a pair of binoculars or a small telescope, because the mountains and craters on its surface are presented very clearly. Even though only half of the Moon's face is illuminated, this is a time when the terminator line which divides the illuminated and unilluminated portions of the Moon's disk is clearly visible.

Along this line, an observer on the Moon would see the Sun rising above the horizon. As it does so, it illuminates the lunar landscape at a low angle, making mountains and crater rims cast long shadows which are easy to see from Earth, even through a modest pair of binoculars.

Seasonal variation

Although the Moon is always separated from the Sun by the same amount – 90° – when it passes first quarter, it is more favourably placed in the early evening sky at some times of year than others.

Specifically, it appears high up in the evening sky around the spring equinox, but much lower towards the horizon around the autumn equinox.

This is because it always lies close to a line across the sky called the ecliptic. This is the line through the zodiacal constellations that the Sun follows through the year, and marks the flat plane in space in which all of the planets circle the Sun.

The altitude at which the Moon appears above the horizon at sunset depends how steeply the line of the ecliptic is inclined to the horizon. If the plane of the ecliptic meet the horizon at a very shallow angle, the Moon will rise or set along a line which is almost parallel to the horizon, and a large separation from the Sun along this line would still only correspond to a very low altitude in the sky.

The inclination of the ecliptic plane to the horizon at London varies between 61° (sunset at the spring equinox) and 15° (sunset at the autumn equinox). On August 18, the ecliptic is inclined at 17° to the western sunset horizon, as shown by the yellow line in the planetarium view above, meaning that on this occasion the Moon is poorly placed for viewing from London.

The Moon's position

At the moment it reaches first quarter, the Moon's distance from the Earth will be 392,000 km. Its exact position will be as follows:

Object Right Ascension Declination Constellation Angular Size
The Moon 15h35m50s -14°12' Libra 30'25"

The coordinates above are given in J2000.0.


The details of this observing event were provided courtesy of In-The-Sky.org

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